Turning Growth Ratio into a Recommended Nitrogen Rate

I’ve said it before, but I’ll say it again: nitrogen is the most important factor to high quality playing surfaces. I’ve never been afraid to admit what I don’t know and in the earlier years of my career, I didn’t know enough about nitrogen, but I’m always trying to learn and get better, whether than advice from friends and colleagues, my own experience, or a combination of both. I pride myself in getting better and becoming more knowledgeable every year.  

Of course, this should be the case after a decade or more of doing something. A person not getting better at their job probably isn’t going to be doing it for long. As an example, I like to use this tweet.

When I took and posted this photo on the old Twitter, I was sure I knew what I was doing. A consultant had once told me that after a one inch or more rainfall, we needed to replace the lost potassium. So, here we were doing exactly that. Micah Woods called me out on it and rather than get defensive, I went about finding out why he thought I was wrong to be making this application. This was the last time we made this application. 

Part of learning and getting better is being humble enough to know when someone knows more than you do. When we recognize our lack of knowledge, we can set about learning something new and getting better. I’m learning about nitrogen uses and efficiency all the time, and it’s largely because I’ve been happy to admit, I didn’t know that much.

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In an earlier post, I wrote about my recent use of the Growth Ratio equation as a way to know exactly how much our turf is growing, while factoring for weather. My aim is to grow the grass as consistently slow as possible through out the golf season, while still maintaining a championship-level surface.  

Why do I believe nitrogen is so important? Simple—too little and your turf becomes dirt, too much and you have soft slow playing conditions—that’s obvious. What I think is less obvious, is that too little nitrogen quickly becomes the right amount and the right amount of nitrogen very quickly becomes too much. 

Get the growth rate just right, using only the necessary amount of nitrogen and one can avoid much of the surface disruption golfers hate.

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Chart 1 below comes from the 2022 season and shows nitrogen amounts (on putting surfaces) accumulated over the season, in three different ways.

1. Gray-the amount of nitrogen the plant will use, as predicted by growth potential (GP).

2. Orange-the amount of nitrogen removed through mowing.

3. Blue-The amount of nitrogen we applied.

Chart 1

My intent, at the beginning of the 2022 season was to apply nitrogen matching the amount GP was predicting the plant would be using. This can be seen in the chart up to mid-August, with blue nearly matching gray. 

In mid-August I begin to observe the greens were too “scuffy”. Basically there was too much leaf tissue-the kind of appearance that might cause one to verticut, or implement some other surface disruption. We did switch from smooth to grooved rollers, but I also decided to reduce the amount of nitrogen applied. This can be seen in the blue stair steps flattening out and moving away from the gray. 

With no disruption, the surfaces improved, the scuffy-ness went away, and I begin to evaluate how we’d apply nitrogen in 2023.

As is typical when I’m doing a deep dive on anything turf related, I conferred with Dr. Micah Woods. Micah’s comments can be summarized as such:

• The nitrogen necessary is probably much closer to N removed by clippings, than N predicted by GP. (We saw this at the end of 2022.)

• Tracking the growth ratio (GR), will tell us if the grass is growing too much, or too little when factoring for weather. You should aim to have an average GR as consistent as possible throughout the season. 

• He sent a formula he felt would help in using the GR to predict/inform the amount of nitrogen we should be applying.

Chart 2 shows the same data for the 2023 golf season. It’s quite obvious to see the difference from 2022. In simple terms, our N approach has gone from matching N predicted by GP, to matching the N harvested from clippings.   

Chart 2

Generally, we match the N harvested from clippings by applying 0.05 lbs/N/M (0.24g/m²), every time we spray putting surfaces. Why this amount? It’s about the amount of N harvested from clippings over a 10-14 day period, which is our typical spray interval.

However, depending on how much the grass is growing, 0.05 might be a little bit too much, or it might not be enough. Luckily, there’s a formula for that. 

I mentioned above that Micah Woods had sent me a formula to help predict nitrogen application amounts, based on growth. In August, I put the formula into play and it looks like this:

0.05(0.3/GR₁₄)

0.05=the amount of N I would normally apply at each spray interval.

0.3=the growth ratio(GR), I’ve determined is the best for our bentgrass putting surfaces.

GR₁₄=The trailing 14-day average growth ratio. 

In my spreadsheet, I have a column in which this formula is calculated every day, based on the daily GR. If the average GR over the past 14 days is above 0.3, the formula suggests a lower N rate. If the average GR is lower, it suggests a higher rate of N.

When it comes time to make an application, I simply look at a cell on my spreadsheet and it tells me an amount of N to apply, that should keep our GR right where we want it. The rate of N, recommended at each application and accumulated through the season, is represented in gold on chart 2.

I’m in the early stages of using this formula, but the results are promising.  

This is pretty heavy subject matter and it’s not necessarily intuitive. I plan to write more about some of the nuance, but for now, I’ll let you digest, and if you feel so inclined, read again.

I’ll finish this post with these thoughts:

I’ve said it already, but I’ll say it again—I’m realizing what I once thought was the right amount of N is too much and what I once thought was too little, seems just right. 

The goal, as with almost everything I’ve written about—maximizing the number of days each season, in which our members experience championship-level conditions. This new way of thinking about nitrogen is something I believe can help us accomplish this goal.